Add-ons
Extra markets and extra JustChat budget can be added to any plan, so you do not have to jump a whole tier for a little more capacity.
Two add-ons, both recurring alongside your subscription and both cancellable separately.
Extra markets
Adds market slots to your current plan. Useful when you are two markets over your limit — buying the slots is usually cheaper than the next tier up, and you keep the rest of your plan as it is.
Each plan has a maximum number of extra markets it can take. Past that point the larger plan is the answer, and it will be the better deal anyway.
JustChat boosters
Adds to your monthly chat budget. Same reasoning: if you consistently run out a week before the period ends, a booster is cheaper than a tier.
There is a limit on how many boosters one subscription can hold.
Adding and removing
Both are managed on the billing page. Changes are prorated automatically and take effect immediately — a market slot you add is usable straight away, and budget you add is available at once.
Removing an add-on that is currently in use will put you over your limit. Existing markets keep running; you will not be able to create new ones until you are back under.
When to move up instead
If you are adding the maximum of both, the next plan is almost certainly cheaper and gives you more users as well. The add-ons exist to smooth the gap between tiers, not to replace them.